Add slow vector initializations lint

[gnieto]

Closes #3237

Add lint to detect slow zero-filled vector initialization. It detects
when a vector is zero-filled with extended with repeat(0).take(len)
or resize(len, 0).
This zero-fillings are usually slower than simply using vec![0; len]

[sinkuu]

Calling this code "zero-filling initialization" may be misleading. with_capacity ends up in Alloc::alloc which does not ensure that the allocated memory is zero filled (unlike alloc_zeroed), right?

[gnieto]

Yes, you are right, is misleading. I'm working on splitting the current lint into two: One for slow initialization and another for the unsafe.
I'll push the change tomorrow.

Thanks!

[gnieto]

I've pushed a change which splits the previous lint into two: One for slow and one for slow initializations.

[phansch]

retriggering CI

[ghost]

This looks good but seems like the it doesn't account for statements between initialization and extension.

For example,

fn do_stuff(vec: &mut Vec<u8>) {
    // ...
}

fn extend_vector_with_manipulations_between() {
    let len = 300;
    let mut vec1:Vec<u8> = Vec::with_capacity(len);
    do_stuff(&mut vec1);
    vec1.extend(repeat(0).take(len));
}

I'm pretty sure that will lead to false positives.
I think it might be best to use the approach used in the almost_swapped lint and just check two consecutive statements.

[gnieto]

Thanks @mikerite for the review.

I'll change the behavior and only check consecutive statements.

[gnieto]

@mikerite I've added a new commit which only checks the following (relevant) expression. I've tried to mimic the style on almost_swapped lint, but there were some edge cases that didn't work.

[ghost]

I wanted to merge this now but I found that don't have write access any more. Hopefully, someone with access will pick this up.

[flip1995]

This needs a rebase and maybe another run of util/dev update_lints.

[gnieto]

Rebased and ran again util/dev update_lints